A large part of the contamination of soil in this country is due to leakage of tanks or dumping of volatile liquid hydrocarbons onto the ground. The liquid hydrocarbons percolate into the soil to form an irregular plume and sometimes migrate to and mix with potable water. It is estimated that over 1/3 of the more than 1 million motor fuel tanks in this country are leaking and over 1/2 of the leaking tanks, leak at a rate in excess of 6 gallons per day. Over 1 million gallons of fuel are lost into the ground each day.
Soil excavation has been the traditional method for decontaminating a site with absorbed hydrocarbon contamination. It is often difficult to assess the full extent of the contamination. Besides being costly, excavating the soil merely changes the location of the contaminated soil. The number of sites for storing hazardous waste is decreasing. The expense and the regulations concerned with transporting the hazardous waste from the contaminated sites to the storage sites makes this an unattractive method of disposal. Current laws make the owner of the waste responsible forever for the stored waste whether it is the contaminated dirt or the spent carbon absorbent.
Soil ventilation is a cost effective way to decontaminate soil. This is effective in locations where the contamination has not reached groundwater. Currently there are two general methods used for remediation of groundwater before it can be discharged into a reinfiltration gallery, sewer or storm drain. These are carbon filtration or air stripping. Carbon filtration is not desirable on highly contaminated sites as the cost of carbon and its associated handling and disposal costs become prohibitive. With air stripping, the cost of carbon is eliminated leaving only replacement costs of packing as the major maintenance expense. However, in areas where emissions are controlled, carbon canisters for air polishing are required. When the soil is highly contaminated with hydrocarbon, the associated costs of carbon and the storage of the contaminated spent carbon again become prohibitive.
One proposal for the elimination of gasoline vapors is to burn the recovered vapors. The level of hydrocarbons recovered in the vapor stream is usually not sufficient to maintain combustion by these vapors alone. Either additional fuel must be added to the vapor to sustain combustion or catalyst must be used to maintain combustion.
A more effective manner of removal of vaporizable hydrocarbon liquid from a subsurface zone is to remove the liquid from the zone by vacuum extraction and combust the liquid hydrocarbon in an internal combustion engine which develops power to power the water lift and extraction pumps and any other energy or heat required to operate the system. If the hydrocarbon liquid is mixed with water the hydrocarbon liquid can be first separated in a spray aeration vacuum unit at high temperature as disclosed in U.S. Pat. No. 4,979,886 entitled Remediation of Combustible Organic Contaminated Water, the disclosure of which is incorporated herein by reference.
The system can be mounted on a skid, tractor or truck bed so that it is fully portable and self-contained. The system can be placed on-site and operated until all contaminates are removed from the soil. When no more vapor is extracted, the engine stops and the site is remediated.
The internal combustion engine on the transportable units have a limited capacity, typically about 80 cubic feet per minute (CFM). However, the vacuum extraction pumps and spray aeration unit can pull much higher flow such as 200-300 CFM. Air quality regulations do not allow the excess volatile hydrocarbons to be vented to atmosphere.
The commonly used approaches for cleaning up the excess volatile hydrocarbons being brought to the surface by the vacuum pump of the spray aeration unit are thermal and catalytic incineration, adsorption and absorption. Absorption and adsorption on carbon or molecular sieves are not cost effective and the sorption media are difficult to regenerate.
Catalysts allow the conversion of VOC's at significantly lower temperatures than thermal incineration and at a much lower operating cost. In addition to lower fuel costs, catalytic incineration units are usually smaller and the lower operating temperature often allows use of less expensive metals of construction, lower capital and maintenance costs and lower NOx production.
However, the catalyst must be operated within controlled temperature ranges. The catalyst inlet temperature is the controlling factor of conversion performance. The onset of conversion is known as the light-off temperature. Conversion rises rapidly with increasing temperatures until it reaches a constant level. This usually occurs at the point that the conversion is controlled by diffusion of the VOC's through the bulk gas film.
In order to be above the steep ascent region and to achieve maximum conversion efficiency, most beds are operated in the diffusion controlled region. However, the temperature in the diffusion controlled region is higher than the temperature of the vadose gas/air mixture feed steam. Also if the temperature of the catalyst bed is raised too quickly, the catalyst can sinter which shortens its life.